The present invention relates to a hydraulically operated unit, particularly a prop, for a roof support in underground operations. The invention also relates to a method of manufacturing such a hydraulically operated unit.
It is known in the prior art to coat piston rods, tubular pistons or inner cylinders composed of steel with chromium, nickel, zinc, paint or a synthetic plastic coating. The thickness of such a corrosion-protective layer lies in the range below 100 microns. These coatings do not, however, provide sufficient protection against aggressive weather and water as well as high impact loads which act upon the inner parts of the working cylinder incorporated in the roof support in underground operations.
In addition to the chemical or organic coating, it has also been proposed to provide the inner parts with thin-walled tubular jackets. The DE-OS No. 2,813,228 discloses a method in accordance with which a cylindrical pipe has a very narrow fit, or in other words with a play, and is fitted onto another pipe or a round material with utilization of force. The fitting takes place in such a manner that the tubular jacket is subjected to a heat treatment and during the heat treatment a continuously acting pulling tension is applied. The expenses needed for the respective device are very high, and a very accurate monitoring is required.
The DE-GM No. 1,898,705 discloses a piston of a hydraulically activated working cylinder which is coated with a thin sheet of corrosion-resistant material. For this purpose the jacket sheet is placed in a foldable sleeve and subsequently is introduced onto the piston provided on its surface with a hardenable synthetic plastic intermediate layer. After hardening of the synthetic plastic intermediate layer, the sleeve can be removed and the piston is provided with the coating.
The DE-OS No. 2,728,226 discloses a proposal in accordance with which the outer surfaces of hollow piston rods are protected with thin-walled fine steel having a wall thickness of 0.2-1 mm. The tubular jacket is produced by longitudinal or spiral welding or by hydraulic pressing onto a mandrel. The placing of the tubular jacket is performed by expanding the carrier tube. The expansion can be carried out by hydraulic pressure or piston pressure. In both cases, an insulation layer between the carrier tube and the tubular jacket is provided. When the insulation layer can be dispensed with or the carrier tube is composed of aluminum and provided with an anodic oxidation layer, the tubular jacket can also be fitted onto the same.
Finally, the DE-AS No. 2,708,929 discloses the proposal in accordance with which the inner cylinder of a mine prop is provided with an outer protective jacket of a fine steel pipe with a wall thickness of 0.2-0.1 mm. It is indicated that the inner cylinder and the protective jacket having the same E modulus can be fixedly connected with one another by pressing. However, it is not disclosed there how the pressing is to be carried out. This reference also does not explain the utilization of the specific properties of the material, with the exception of the same E modulus.